Thrombosis: haemostasis in the wrong place

Question 1

What is the primary and secondary function of haemostasis?

Answer 1

1. Primary haemostasis is aggregation of platelets.
2. Secondary is conversion of fibrinogen into fibrin.

The surface of platelets is an important component of the process

Question 2

Give an overview of the primary and secondary haemostasis in response to tissue damage

Answer 2

Primary = platelet ---> activated platelet
Secondary = Fibrinogen ------> Fibrin 

These lead to clotting

Question 3

Describe how a clot is formed (3)

Answer 3

1. Aggregation of platelets.
2. Fibrinogen converted into fibrin mesh by thrombin which is a protease.
3. Thrombin is itself converted from prothrombin which is the culmination of a cascade of similar activation steps.

Question 4

What process reverses thrombosis?

Answer 4

Fibrinolysis

Question 5

Describe how arterial and venous thrombosis are different

Answer 5

Arterial thrombosis

1. Mostly result from atheroma rupture or damage to the endothelium (eg. MI, stroke).
2. Platelet-rich ‘white’ thrombosis. Mostly primary.
3. May block downstream arteries.

Venous thrombosis

1. Often results from stasis or a hyper-coagulant state (eg DVT).
2. Platelet-poor ‘red’ thrombus. Mostly secondary.
3. May move to lungs.

Question 6

How do cells maintain the correct balance of between thrombosis (coagulation) and fibrinolysis

Answer 6

1. Endothelial cells express various factors, inhibiting coagulation:

Nitric Oxide - inhibits platelets
Antithrombin and Heparan - inhibits clotting
Prostaglandin I2, (inhibits platelets)
Tissue plasminogen activator - converts plasminogen to plasmin which converts a fibirin clot to a D-dimer by undergoing fibrinolysis

2. If endothelial cells become damaged or inflamed they may favour coagulation, subendothelial cells if disturbed release tissue factor or Von Williebrand factor

Question 7

What does Virchow’s triad consist of?

Answer 7

Stasis - Static blood lacks kinetic energy and tends to clot

Endothelial damage - eg. surgery or cannula

Hyper-coagulant state - eg. Infection/sepsis; genetic predisposition; drugs (eg. HRT)

Question 8

What are the roles of valves inflow in veins?

What does contraction of veins cause?

What does blood tend to do around valves?

Answer 8

• Valves in veins prevent backflow of blood
• Contraction of nearby muscles squashes veins, acting as a pump to return blood to the heart
• Blood tends to eddy around valves increasing risk of stasis

Question 9

What are the 4 fates of thrombosis?

Answer 9

1. Resolution i.e thrombolysis
2. Embolism i.e moves to another location and blocks vessels
3. Organised i.e becomes covered by endothelium
4. Recanalised and organised

Question 10

Give an overview of proximal DVT

Answer 10

Higher risk of embolism and post-thrombotic syndrome (pain, swelling and ulcers) Occurs between the external iliac arteries and Popliteal vein

Question 11

Give an overview distal DVT

Answer 11

Rarely caused by pulmonary embolism
Rarely cause post-thrombotic syndrome
Below the tibial vein

Question 12

What is Post-thrombotic syndrome?

Answer 12

• Inflammation along with damage to the venous valves from the thrombus itself.
• Valvular incompetence combined with persistent venous obstruction inducing a rupture of small superficial veins, subcutaneous haemorrhage and an increase of tissue permeability.
• Pain, swelling, discoloration, and even ulceration follow.

Question 13

What is the fate of venous thrombosis - lungs

Answer 13

If the clot goes back to the heart, it can be asymptomatic or death. It can pass through the heart and pass out to the lungs, it will then block a small vessel in the lungs. VQ mis-match – later in the lungs. A tiny area of the blood becomes starved and will then be degraded. It can be more serious and cause hypoxia. Sometimes can also block pulmonary arteries and cause death.

Question 14

Describe the process of platelet adherence (3)

Answer 14

• Von Willebrand factor on subendothelial cells activates platelets.
• Circulating Von Willebrand factor may bind to exposed subendothelial cells
• Activated endothelial cells can also express von Willebrand factor

Question 15

Describe the process of platelet activation (3)

Answer 15

• Activated platelets release Thromboxane A2 (TxA2) & Adenosine diphosphate (ADP) which induce receptors for fibrinogen (GPIIb/IIIa).
• These bind to receptors on adjacent platelets and increase expression of the glycoprotein complex GPIIb/IIIa.
• Platelets can also be activated by thrombin, collagen and many other mediators.

Question 16

Describe the process of platelet aggregation (2)

Answer 16

• Fibrinogen acts as a tether, holding platelets together. This is aggregation.
• Fibrinogen is the soluble precursor to fibrin and is in the circulation.

Question 17

Describe the process platelet substrate for coagulation

Answer 17

• Once a clump of platelets aggregate they form a negatively charged surface.
• Coagulation involves the conversion of fibrinogen to fibrin and then crosslinking of the fibrin clot.

Question 18

What is extrinsic and intrinsic pathways

Answer 18

Extrinsic pathway which is probably the most important in Vivo.

Intrinsic pathway forms the basis of laboratory tests of coagulation -probably not so important in vivo but there is some interaction with the extrinsic.

Question 19

What is the common pathway that forms fibrin?

What does Factor Xa do?

Answer 19

Once the extrinsic and intrinsic pathways join together:

1. Factor Xa cleaves prothrombin to form thrombin
2. Thrombin is a protease that cleaves fibrinogen into fibrin. This is an insoluble molecule
   3, Fibrinogen promotes blood clotting by forming bridges between and activating blood platelets through binding to their GPIIb/ IIIa surface membrane fibrinogen receptor.

Question 20

Describe Tenase & prothrombinase

Answer 20

Thrombin cleaves factors V and VIII to give Va & VIIIa.

Va & VIIIa together with plasma Ca2+ form two complexes

Tenase complex: VIIIa + IXa → Xa

Prothrombinase complex: Va + Xa → Thrombin

The complexes assemble on the charged phospholipid surfaces of activated platelets

Question 21

Describe the structure of the GLA domain and what is it converted to?

What does GLA then do?

What is the carboxylation of glutamic acids dependant on?

What inhibits production of potassium ions?

Answer 21

GLA domain is 10-12 glutamic acids in the N-terminus of the molecule converted to gamma-carboxyglutamic acid (Gla).

The GLA domain binds calcium ions by chelating them between twocarboxylicresidues. Domain present on prothrombin, VII, IX, and X.

The carboxylation of glutamic acid to create the GLA domain is vitamin K-dependent.

Warfarin inhibits an enzyme involved in vitamin K production.
So is an effective anticoagulant.

Question 22

What happens when enough thrombin has been generated?

Answer 22

Once enough thrombin has been generated, XIII is activated, which crosslinks the fibrin fibres into a solid clot.

Question 23

Describe what happens in the extrinsic pathway

Answer 23

• Begins in the vessel wall. Damaged endothelial cells will release factor III (tissue factor), and the greater the amount of damage, the more is released.
• This combines with calcium on negatively charged platelet surface and activates factor VII.
• Once activated, VIIa activates Xa and the common pathway is initiated.
• The VIIa-tissue factor complex can be quickly inactivated by antithrombin III in vivo.

Question 24

Describe what happens in the intrinsic pathway

Answer 24

• Begins in the blood stream. It is basically activated when blood is exposed to collagen (or other damaged surfaces).
• Activated when you put blood onto a charged surface such as glass.
• Defects in the factors of the extrinsic pathway have far larger physiological effects than mutations in the enzymes of the intrinsic pathway.
• Laboratory tests distinguish between activating the intrinsic or extrinsic pathways in order to assess where a defect may be.

Question 25

Fibrinolysis - Describe the use of tissue plasminogen activator

and how do they work?

Answer 25

Some of these thrombolytic agents such as tissue plasminogen activator and related compounds are used to treat strokes and myocardial infarctions.

Tissue plasminogen activator is a serine protease found on endothelial cells which catalyses the activation of circulating plasminogen into plasmin.

Plasmin catalyses the breakdown of cross linked fibrin clot into fragments called D-dimers.

Question 26

What is antithrombin and how is it activated?

Answer 26

Antithrombin (AT) is a small protein molecule made by the liver that circulates in the plasma.

Heparan is expressed by endothelial cells and binds to the enzyme inhibitor antithrombin III (AT), causing a conformational change that results in AT activation.

The activated AT then inactivates thrombin, factor Xa, factor VII and other components of the clotting